Two broods of periodical cicadas will emerge at the same time in May (Credit: CC-BY-SA-2.0)

Sixteen US states, stretching from Maryland to Oklahoma and Illinois to Alabama, are bracing for the arrival of trillions of noisy cicadas. What makes the event especially rare is that two broods are emerging simultaneously. Brood XIX, comprising four species that surfaces every 13 years, and Brood XIII, consisting of three species that appears every 17 years. This unusual coincidence last occurred 221 years ago, in 1803, and won't happen again until 2245.

The insects have already begun to emerge in some areas. However, their numbers are expected to peak for a two-to-three-week period starting mid-May. Experts believe that the two broods will come close in many states. However, they will only overlap in Illinois and Iowa.

What are periodical cicadas?

Periodical cicadas spend most of their lives underground, feeding off sap from tree roots. The black, shrimp-sized bugs with red, beady eyes emerge every 13 or 17 years to reproduce and die soon after.

Why do cicadas come out every 13 or 17 years?

Researchers have an interesting theory about why cicadas come out every 13 or 17 years. They believe that if the insects appeared after an even number of years, like 12 or 16, they would become predictable prey for predators with 2, 4, and 8-year lifespans. However, since 13 and 17 are prime numbers, any predator dependent on the cicadas must match those lifespans. As for how the cicadas know when it is time to emerge? That is a secret known only to the insects themselves!

The two cicada broods are not expected to overlap (Credit: CC-BY-SA-2.0)

Are cicadas dangerous?

Cicadas are harmless to humans. But their massive swarms can be daunting and even a nuisance since they seem to appear everywhere. Furthermore, the loud, high-pitched drone of male cicadas, which reaches 90 decibels as they try to attract females, can be extremely annoying.

The slits created to lay cicada eggs can damage young plants (Credit: G. Edward Johnson/ CC-BY-SA-3.0/ Wikimedia Commons)

Noise aside, the insects can cause significant damage to young trees. Female cicadas seek out slender twigs or vines to lay their eggs. The incisions they make to deposit their eggs often sever the food supply, causing the branch to wilt. Mature trees can withstand this damage. However, young fruit or nursery trees get stunted or killed. Experts suggest covering them with nets during the cicada onslaught.

The adult cicadas die soon after the eggs are laid. Their bodies provide a rare feast for animals and nitrogen for growing trees. The eggs hatch in 4-6 weeks, and a new generation of cicadas burrows into the soil to live underground for the next 13 or 17 years.